[1] Normally, the nervous system releases stress signals (e.g. norepinephrine) in certain situations as a defense mechanism to protect one from perceived dangers.
[2] In hypervigilance, there is a perpetual scanning of the environment to search for sights, sounds, people, behaviors, smells, or anything else that is reminiscent of activity, threat or trauma.
They might overreact to loud and unexpected noises, exhibit an overactive startle response or become agitated in highly crowded or noisy environments.
[7] Sustained states of hypervigilance, lasting for a decade or more, may lead to higher sensitivity to disturbances in their local environment, and an inability to tolerate large gatherings or groups.
After resolution of the situation demanding their attention, people exhibiting hypervigilance may be exhausted and require time before returning to normal activities.